Contribution to a Sustainable Society: Biosorption of Precious Metals Using the Microalga Galdieria.

The red microalga Galdieria sp. is an extremophile that inhabits acidic hot sulphur springs and grows heterotrophically to a high cell density. These characteristics make Galdieria suitable for commercial applications as stable mass production is the key to success in the algae business. Galdieria has great potential as a precious metal adsorbent to provide a sustainable, efficient and environmentally benign method for urban mining and artisanal small-scale gold mining. The efficiency and selectivity in capturing precious metals, gold and palladium from metal solutions by a Galdieria-derived adsorbent was assessed relative to commercially used adsorbents, ion exchange resin and activated charcoal. As it is only the surface of Galdieria cells that affect metal adsorption, the cell content was analysed to determine the manner of utilisation of those metabolites. Galdieria was shown to be protein-rich and contain beneficial metabolites, the levels of which could shift depending on the growth conditions. Separating the cell content from the adsorbent could improve the adsorption efficiency and reduce CO2 emissions during the metal collection process. The commercial applications of Galdieria appear promising: growth is quick and dense; the precious metal adsorption capacity is highly efficient and selective in acidic conditions, especially at low metal concentrations; and the cell content is nutrient-rich.

Augmentation in bioleaching potential of indigenous Bacillus sp. ISO1 for metals recovery from waste computer-printed circuit boards.

The bio-cyanidation process of various cyanogenic microorganisms is found to be a sustainable and effective method for metals recovery from primary and secondary sources. This process has surpassed the limitations of the chemical cyanide treatment process; thus, prioritized as a promising approach for e-waste "urban mining" strategies. The main focus of the study was to enhance the bioleaching capacity of indigenous Bacillus sp. ISO1 and to implement optimized parameters in large-scale bioleaching operations. The assessment of various amino acids unveiled that like other cyanogenic microorganisms Bacillus sp. ISO1 also preferred glycine as a prime precursor for cyanide synthesis, as maximum metal solubilization was achieved with glycine amino acid. Other amino acids influenced the bacterial growth but not significantly affected the biocyanidation process. The evaluation and optimization of methionine as a lixiviant stimulator demonstrated that the addition of 1 mg/L methionine effectively enhance the production of glycine-utilizing cyanide lixiviant, that led to a significant solubilization of Cu (86%), Au (75%), and Ag (63%) metals. Furthermore, the kinetics of metal solubilization and operating conditions were explored at increased volume (i.e., 3 L working volume) of bioleaching medium to assess the industrial scale potential of this potent bacterial strain with optimized parameters such as temperature, pH, pulp density, and inoculum size. The significant recovery of Cu (˃ 60%) and other metals at this substantial volume suggested the implementation of a bioleaching process with this potent bacterial strain at industrial scale operations.

Precious metal recovery from electronic waste by a porous porphyrin polymer.

Urban mining of precious metals from electronic waste, such as printed circuit boards (PCB), is not yet feasible because of the lengthy isolation process, health risks, and environmental impact. Although porous polymers are particularly effective toward the capture of metal contaminants, those with porphyrin linkers have not yet been considered for precious metal recovery, despite their potential. Here, we report a porous porphyrin polymer that captures precious metals quantitatively from PCB leachate even in the presence of 63 elements from the Periodic Table. The nanoporous polymer is synthesized in two steps from widely available monomers without the need for costly catalysts and can be scaled up without loss of activity. Through a reductive capture mechanism, gold is recovered with 10 times the theoretical limit, reaching a record 1.62 g/g. With 99% uptake taking place in the first 30 min, the metal adsorbed to the porous polymer can be desorbed rapidly and reused for repetitive batches. Density functional theory (DFT) calculations indicate that energetically favorable multinuclear-Au binding enhances adsorption as clusters, leading to rapid capture, while Pt capture remains predominantly at single porphyrin sites.

Evaluation of the recycling potential of obsolete mobile phones through secondary material resources identification: A comprehensive characterization study.

The growing concern over the management of e-wastes has generated an interest in the recovery of resources from these wastes under the concept of urban mining and circular economy. However, in the absence of accurate knowledge of the physico-chemical compositional structure of these wastes makes the recycling process difficult. Thus, the present study conducted a recycling-oriented characterization of waste mobile phones (WMPs) for the identification of secondary materials and estimated their recycling potential. The characterization was performed using ICP-OES and FTIR techniques after dismantling WMPs for the determination of elemental composition and the polymeric fractions respectively. Dismantling of the WMPs revealed that enclosures, batteries, display modules, and PCBs consist of 35.33 wt%, 28.9 wt%, 19.44 wt%, and 16.31 wt% respectively. Of these components, PCBs constitute the highest economic recovery potential with an estimated potential revenue generation of more than 50,000 US $ per ton of waste PCBs. Copper showed the highest recovery potential (234.39 tons/year) with an economic value of approximately 3317 US $/ton of WPCBs followed by Sn (27.37 tons/year) and Ni (24.64 tons/year). Among different precious metals, Au was found to have the highest percentage of economic value (76.22%) followed by Pd (8.16%) and Ag (3.13%). The display modules and enclosures were found to have relatively lower contributions than WPCBs in the overall recycling potential due to lower metal contents and mixed polymeric fractions. The findings in the study indicate that WMPs could serve as a promising new source for sustainable secondary mining of rare and valuable metals. Further, the study will help the policymakers in designing effective e-waste management strategies through the promotion of sustainable recovery of materials.

Assessment of the recycling potential of valuable metals by mapping the elemental composition in discarded light-emitting diodes (LEDs).

Electronic waste (e-waste) is the world's fastest-growing type of waste, with lighting accounting for 9% of the total. Light-emitting diodes (LEDs) are composed of the most concentrated critical elements (Ag and Au) and recovery of these metals could generate economic benefits and reduce the burdens of environmental pollution; nevertheless, the absence of information about their composition currently presents a challenge in recycling these metals with minimal prospects for recovery. This study assessed the distribution and variation of elemental concentrations of 16 different elements in three generations of LEDs (12 different LED units): sub-mounted-device (SMD #10), chip-on-board (COB #1), and positive-intrinsic-negative (PIN #1). The SMD LEDs contained a considerable amount of Au with a median average concentration of 1204 mg/kg (ranging from 323 - 3687 mg/kg), which was similar to that of COB (1550 mg/kg), but higher than that of PIN LED (175 mg/kg). Based on the total threshold limiting concentration (TTLC), the Cu levels (605,823 mg/kg) in the SMD package exceeded the regulatory limits (2500 mg/kg). Concentrations of the hazardous elements Cr (29 mg/kg), Pb (12 mg/kg), Cd (0.1 mg/kg), and As (1 mg/kg) in the LED packages were within the regulatory limits. To recycle precious metals and other technological metals, a well-organized and dedicated optimized assessment of the value of metals is required especially in accordance with the concept of criticality and recyclability. Two factors, i.e., a high resource index (RI) and technology index (TI), suggest the importance of waste to the economy and has a significant potential for recycling with less processing burdens. Present findings indicated that the COB and a few of the studied SMD LEDs (3020, 4014, 5630, and 7020), exhibit high criticality and recyclability. For the RI and TI index, the contribution of metals such as Cu, Fe, Al, and Au were dominant. These findings can serve as a reference for the development of a viable approach for the recycling and recovery of targeted metals from LED e-waste.

c-Si PV module recycling: Analysis of the use of a mechanical pre-treatment to reduce the environmental impact of thermal treatment and enhance materials recovery.

The current increase in the use of photovoltaic (PV) energy demands the search for solutions to recycle end-of-life modules. This study evaluated the use of a mechanical pre-treatment in the thermal recycling of c-Si crystalline PV modules, which were submitted to recycling routes to separate and concentrate the materials of interest. The first route was constituted by only thermal treatment, and the second route was constituted by a mechanical pre-treatment to remove the polymers from the backsheet, and subsequent thermal treatment. The exclusively thermal route was performed at 500°C, varying dwell times between 30 and 120 minutes in the furnace. In this route, the best results were obtained in 90 minutes, with a maximum degradation of 68% of the polymeric mass. In route 2, a micro-grinder rotary tool was used to remove the polymers from the backsheet and, subsequently, thermal treatment performed at 500°C, with dwell times in the furnace ranging between 5 and 30 minutes. The mechanical pre-treatment removed about 10.32 ± 0.92% of the mass of the laminate PV module. By this route, only 20 minutes of thermal treatment were needed for the total decomposition of the polymers, that is, a reduction of 78% in the oven time. With route 2, it was possible to obtain a concentrate with 30 times more silver than the PV laminate and 40 times more than a high-concentration ore. Furthermore, with route 2 it was possible to reduce the environmental impact of heat treatment and energy consumption.

Decadal Journey of E-Waste Recycling: What Has It Achieved?

E-waste recycling has been a hot topic around the world. This Feature revisits the issues raised by our previous Feature 10 years ago, i.e., the environmental, economic, and social benefits of e-waste recycling, using China as an example. The decadal journey of e-waste recycling has witnessed a giant leap from haphazard disposal initially to regulated disassembly presently. Specific successful stories include cleaned environment and reduced human exposure, significant advantages of urban mining over mineral mining, additional employment opportunities, and improved legislation system related to e-waste recycling. Strict legislation systems related to e-waste management based on the principle of Extended Producer Responsibility are key to the sustainable development of the e-waste recycling sector in China. The experiences and lessons learned in China would provide valuable guidelines for other developing countries.

Olive-tree polyphenols and urban mining. A greener alternative for the recovery of valuable metals from scrap printed circuit boards.

Recycling printed circuit boards (PCBs) is becoming a source of precious metals and an alternative to conventional mining. This phenomenon is now known as "urban mining." In this work, a polyphenols-rich plant extract has been obtained from olive-tree leaves, and its ability to contribute to reducing four metals, namely, Ag, Cu, Cr, and Sn, that are present in scrap PCBs has been studied. Three reductants (NaBH4, Fe°, and the olive-tree leaves extract) have been used to recover these valuable metals. An attempt has been made to minimize the concentration of the first two, replacing them with a natural, cheaper, and less toxic reductant. To achieve this goal, a computer-assisted factorial, composed, centered, orthogonal, and rotatable statistical design of experiments (FCCORD) has been used to build the experimental matrix to be carried out in the laboratory and, next, for the statistical treatment of the results. The results show that it is possible to achieve only a partial recovery of the four metals (silver, copper, chromium, and tin) from PCBs leachates by using sodium borohydride, iron, and the extract separately. In other words, none of these three reductants alone can completely remove any of the four metals in the leachate. Nevertheless, using the statistical design of experiments, the total recovery of the four metals has been achieved by combining the three reductants in the appropriate concentrations. Hence, polyphenols-rich plant extracts in general and olive-tree leaves extract in particular can be regarded as promising coadjuvants in the rising field of urban mining.

Sustainable approach for valorization of solid wastes as a secondary resource through urban mining.

The incessant population has increased the production and consumption of plastics, paper, metals, and organic materials, which are discarded as solid waste after their end of life. The accumulation of these wastes has created growing concerns all over the world. However, conventional methods of solid waste management i.e., direct combustion and landfilling have caused several negative impacts on the environment (releasing toxic chemicals and greenhouse gases, huge land use) besides affecting human health. Therefore, it is requisite to determine sustainable alternative technologies that not only help in mitigating environmental issues but also increase the economic value of the discarded solid wastes. This process is known as urban mining where waste is converted into secondary resources and thereby conserves the natural primary resources. Thus, this review highlights the technological advancements in the valorization process of discarded wastes and their sustainable utilization. We also discussed several limitations of the existing urban mining processes and further the feasibility of valorization techniques was critically analyzed from a techno-economical perspective. This paper recommends a novel sustainable model based on the circular economy concept, where waste is urban mined and recovered as a secondary resource to support the united nations sustainable development goals (SDGs). The implementation of this model will ultimately help the developing countries to achieve the target of SDGs 11, 12, and 14.

Strategies for improving the environmental performance of nickel production in China: Insight into a life cycle assessment.

Nickel is a critical metal for global low-carbon energy transition, but its production processes require massive energy inputs and emit large amounts of pollutants. This study constructed life cycle inventories of the mainstream electrolytic nickel production chains in China at the industrial level and subsequently evaluated their environmental performance via a regionalised life cycle impact assessment method. Results show that environmental indicator results of the electrolytic nickel production from the leaching electrowinning method were 17.7%-40.2% lower than those from the grind and flotation electrolytic method. At the endpoint level, the nickel mining and beneficiation stages contributed 54.7%-65.91% of human health damage, 83.0%-84.7% of ecosystem quality damage and 80.8%-83.7% of resources damage. The key processes, including direct processes, cement input and energy consumption (e.g., electricity and coal), accounted for more than 62.1% of the impacts in the key midpoint categories. The potential environmental damage of China's nickel mining and beneficiation industry increased by 29.2% from 2010 to 2018 because of the growing trend of nickel ore demand. In the case that China's nickel metal recovery rate reaches the global average level, then approximately 3.83 × 102 Daly of human health damage, 59.83 Species·year of ecosystem quality damage and 1.64 × 108 $ of resources damage can be avoided annually. Strategies for promoting the full assimilation of renewable electricity, applying the clinker-free cemented backfill materials in the mining process, precious recovery by bioleaching from tailings and reusing waste rock as building materials are recommended. Meanwhile, extended producer responsibility should to be comprehensively implemented in the nickel-related industries to alleviate the environmental implications and nickel supply pressures from geo-mining.

Forecasting quantities of critical raw materials in obsolete feature and smart phones in Greece: A path to circular economy.

Mobile phones represent an ever-increasing waste stream due to the increasing ownership and short lifetime. In particular, smartphones are among the most valuable e-waste because of their extremely high content of numerous key metals, specifically in the printed circuit board and magnets. As feature and smart phones contain different key metals at different concentrations, it is important to distinguish between the two phone types to make reliable estimations. This study presents estimations of obsolete mobile phones quantities, generated in Greece in 1995-2035 and the Critical Raw Materials (CRMs) and Precious Metals (PMs) embedded in them, making a differentiation between feature and smart phones. The dynamic material flow analysis is adopted, the lifespan is evaluated by the Weibull distribution and future sales are predicted by the logistic model incorporating phases of growth, saturation and decline. Then, the future wastes are predicted by the Market Supply A model. According to the results, the generation of obsolete smartphones is constantly increasing, while the waste flows of feature phones are declining. Efficient recycling of obsolete phones (1995-2020) can cover the demand for key metals (Au, Pd, Co) in the new smartphones for more than a decade in Greece, while the demand for Ag, Sb, Si, Zn, Be, Ti will be covered for more than 15 years. In 2020-2035 the accumulated amounts of CRMs and PMs, only from the smartphone waste, will be 1292.02 and 14.11 tonnes, respectively. The findings can contribute to the management of a valuable e-waste category closing the loop between resources-products-wastes.

Boron recovery from desalination seawater brines by selective ion exchange resins.

The European Union (EU) depends on third markets to supply many important raw materials. Increasing the circularity of critical raw materials within the EU is important not only from an environmental perspective, but also as a competitive advantage for the EU economy. In the case of boron, the EU's import dependency is about 100%. This work aims to evaluate the boron recovery from seawater desalination plants (SWDP) brines using ion-exchange resins in a circular economy approach. Commercial boron selective resins Purolite S108, DIAION CRB03 and CRB05 were tested and compared on batch and dynamic experiments. Thermodynamic and kinetic experiments were performed, and results were fitted by linear and non-linear models. After a comparison, results showed a good fit to the Langmuir isotherm and the pseudo-second order model, respectively, for all the commercial resins tested. The DIAION CRB03 resin presented higher sorption capacity and percentage of boron sorbed than the other resins and was selected as the best option for boron recovery from SWDP brine. Dynamic experiments in fixed bed column using DIAION CRB03 resulted in a sorption capacity of 13 mg/g of resin, a boron recovery of 98% and a concentration factor of 30, for an initial boron concentration of 50 mg/L. In addition, an economic analysis was carried out as a preliminary estimate of the revenues obtained from the production of boric acid from the brine produced by El Prat desalination plant.

The use of urban biowaste and excavated soil in the construction sector: A literature review.

Soil has been used as building material for thousands of years with a decrease in popularity after the industrial revolution. Nowadays, there is a growing interest in the implementation of unfired soil-based building solutions for their low environmental impact, performances and availability. Traditional soil construction techniques have recurrently included vegetal fibres to enhance soil performance and recent studies highlight a predominant use of agro- and non-agro-waste for unfired soil construction. The article reviews the state-of-the-art of the use of excavated soil and biowaste in the construction industry including a novel focus on urban-only waste and on building technologies using the integration of these two secondary construction material flows. Our literature review highlights a lack of references about the joint use of these secondary resources. Finally, future research orientations are suggested to promote their implementation in the building sector, which could improve urban waste management.

Issues and solutions of electronic waste urban mining for circular economy transition: An Indian context.

The rapid consumption of advanced e-products has intensified problems for the linear economy; constantly diminishing natural resources employed in production processes have created a need of recycle and reuse. Although the transition to a circular economy proposes to end the loop of e-products, it needs the application of processes such as urban mining to recover resources as secondary raw material. The present study intends to examine the issues and challenges of electronic waste urban mining (EWUM) in India that need to be assessed for the development of a sustainable economy. To accomplish this, the current study employs integrated Multi-Criteria-Decision making methods (MCDM). Step-Wise Weight Assessment Ratio Analysis (SWARA) is used to prioritize issues and their possible solutions with Weighted Assessment Sum Product Assessment (WASPAS) methods introduced to explore these challenges and provide solutions for managing EWUM. There is an immediate need to acknowledge the issues confronted by stakeholders in urban mining processes for successful transition to a circular economy. A better understanding of the issues will help policy makers and decision makers to implement best practices to enhance the urban mining process in India. This study has shown that socio-economic (SE) issues are the most critical issues in EWUM in India. The possible solutions that would have most impact are to enhance awareness campaigns for people to educate themselves regarding e-waste, train staff to handle safe disposal of e-waste and produce eco-friendly electronic products.

Clean construction and demolition waste material cycles through optimised pre-demolition waste audit documentation: A review on building material assessment tools.

Waste from the construction sector poses huge challenges for sustainable waste management. This is not only due to the vast amount of waste produced in construction and demolition activities, but also due to pollutants potentially contained in these products. Subject to these conditions, waste management must ensure recovery of as many resources as possible, while making sure to keep material loops clean. This demanding task requires more knowledge about the existing building stock and an adaptation of current demolition processes. Innovative technologies, such as Building Information Modelling, or modern frameworks, such as Geographic Information Systems, offer a high potential to synoptically provide stock material information for future demolition activities for individual objects to be deconstructed as well as for whole cities as a basis for managing the anthropogenic stock and potential urban mining. Suitable methods of data collection allow for acquiring the desired input for the generation of building stock models enriched with demolition-related information. With the latter, selective deconstruction strategies as well as appropriate waste stream routing agendas can be planned and executed, thereby securing safety at work during the demolition process itself and a waste stream routing according to the waste hierarchy. This review article gives an overview of currently deployed building material assessment tools (data capture and visualisation), both a prerequisite for improved information on materials and geometry (and thereby mass/volume). In addition, this article describes workflows employable for the purpose of urban mining in end-of-life buildings, of which one holistic approach will be described in depth.

Manganese and iron recovery from groundwater treatment sludge by reductive acid leaching and hydroxide precipitation.

In this study, the recovery of manganese (Mn) and iron (Fe) from groundwater treatment sludge through reductive acid leaching and hydroxide precipitation was investigated. Maximum leached Mn (100%) was obtained using sulfuric acid and hydrogen peroxide at 25 °C. Leached Mn and Fe decreased with the increase in the solid-liquid ratio. Leaching time had minimal effect on Mn and Fe leaching beyond 5 min, while agitation rate had minimal effect beyond 150 rpm. At 25 °C, the rate-limiting step of Mn leaching was diffusion through inert solid components of the sludge, composed mainly of insoluble sand particles. Fe leaching was governed by diffusion through the insoluble components of the sludge, including the unreacted manganese dioxide (MnO2). Maximum precipitation of Fe and separation from Mn in the leachate through addition of potassium hydroxide occurred at pH 4.0. The results demonstrated that reductive acid leaching and hydroxide precipitation is an effective means of recovering Mn and Fe from groundwater treatment sludge. The applicability of the recovered Mn for nickel ion removal from aqueous solution was also explored in the study. Highest nickel ion uptake by the MnO2 synthesized from the recovered Mn was at 111.67 mg g-1, even exceeding the adsorption capacities of previously studied nickel adsorbents.

Global demand for rare earth resources and strategies for green mining.

Rare earth elements (REEs) are essential raw materials for emerging renewable energy resources and 'smart' electronic devices. Global REE demand is slated to grow at an annual rate of 5% by 2020. This high growth rate will require a steady supply base of REEs in the long run. At present, China is responsible for 85% of global rare earth oxide (REO) production. To overcome this monopolistic supply situation, new strategies and investments are necessary to satisfy domestic supply demands. Concurrently, environmental, economic, and social problems arising from REE mining must be addressed. There is an urgent need to develop efficient REE recycling techniques from end-of-life products, technologies to minimize the amount of REEs required per unit device, and methods to recover them from fly ash or fossil fuel-burning wastes.

Potential of metal recovery from printed circuit boards in Brazil.

In this study, the generation of domestic waste electrical and electronic equipment (WEEE) and waste printed circuit board (WPCB) were estimated, from 2015 to 2030. Based on the number of EEE put on the Brazilian market, the possession rate in the Brazilian households and obsolescence amounts of five EEE types were estimated using time series. The results show that, between 2015 and 2030, the quantity of WEEE generated per year will increase from 131.87 kt to 195.22 kt. In this period, WPCB generation will stay around 10% of WEEE generation. Additionally, this study shows that the urban mining potential of the materials recoverable from WPCB can be an important revenue source, with environmental benefits deriving from energy savings and a reduction in CO2 emissions. The results of this study provide a quantitative basis that may help decision makers develop strategic policies for WEEE management, considering material circularity.

Copper recovery by solvent extraction for nanoparticle synthesis from waste motherboards.

Printed circuit boards, which make up part of waste from electrical and electronic equipment, contain elements that can be economically reused, such as copper, silver, gold, and nickel, as well as metals that are harmful to the environment and health, such as lead, mercury, and cadmium. Thus, through recycling this scrap, materials that would otherwise be discarded can be reinserted as secondary raw materials to produce new consumer goods through urban mining. In this context, the synthesis of nanoparticles shows promise as it allows the reinsertion of these materials in the manufacture of new products. Therefore, this study used obsolete computer motherboards as a secondary material to obtain copper to produce nanoparticles of this metal. From a solution based on the leach liquor of this scrap, a purification route using solvent extraction was defined and applied to the real leach liquor. Applying the hydroxyoxime extractant at a dilution of 20% (v/v) in kerosene, A/O of 1/1, 298 K, and 0.25 h of contact during extraction, and stripping in H2SO4 (2 M), 298 K, 0.25 h, W/O ratio of 3/1, and two theoretical countercurrent stages, a solution containing more than 95% of the copper in the leach liquor could be obtained with less than 1% of contaminants. From this purified liquor, nanoparticles containing copper and metallic copper oxides and hydroxides were produced, with an average size of 84 nm, at pH 11, 3 h of hot stirring, volume of 0.015 L of ascorbic acid (0.50 M) and 0.015 L of precursor solution (0.03 M Cu), and temperature (343 K).

Comparative life cycle assessment of copper and gold recovery from waste printed circuit boards: Pyrometallurgy, chemical leaching and bioleaching.

Printed circuit boards (PCBs) make up a substantial amount of electronic waste (e-waste) generated annually. Waste PCBs contain high quantities of copper and gold in comparison to natural ores. As such, "urban mining" of waste PCBs to recover these metals is of commercial interest. In this work, we used life cycle assessment to compare the environmental impact of four copper and gold recovery processes. We evaluated pyrometallurgy, chemical leaching, and bioleaching, as well as a hybrid leaching process that uses bioleaching to recover copper and chemical leaching to recover gold. Furthermore, we considered differences in environmental impact based on differences in electricity sources. If electricity comes from fossil fuels, the pyrometallurgical process results in the lowest environmental impact in all impact categories studied. If electricity comes from carbon-free sources, the pyrometallurgical process results in the lowest environmental impact in all categories studied except global warming, where the hybrid leaching process results in the lowest impact. In all cases, metal recovery from waste PCBs leads to lower environmental impact than primary metal production. Our goal is to guide e-waste recyclers towards more environmentally sustainable metal recovery processes and to provide knowledge gaps in the field to guide future research.